The present invention pertains to microlithography (transfer of a pattern, defined on a reticle or mask, to a sensitive substrate) performed using a charged particle beam (e.g., electron beam or ion beam). Microlithography is a key technique used in the manufacture of microelectronic devices such as integrated circuits and displays. More specifically, the invention pertains to magnetically shielding the charged particle beam from adverse influences of magnetic fields generated by linear motors used to support and move a reticle stage and a substrate stage during charged-particle-beam microlithography.
The ongoing push for increased integration in microelectronic devices has resulted in a current need for being able to transfer patterns having a minimum line width of less than 100 nm. Resolution at this level cannot be achieved using xe2x80x9copticalxe2x80x9d microlithography (i.e., microlithography performed using a light beam, typically a deep-ultraviolet light beam). Charged-particle-beam (CPB) microlithography is an attractive candidate technique for achieving such resolution, for essentially the same reasons that electron microscopy achieves better resolution than light microscopy.
Unfortunately, using current technology, CPB microlithography cannot be performed satisfactorily by exposing an entire reticle pattern (for an entire layer of a xe2x80x9cdiexe2x80x9d or xe2x80x9cchipxe2x80x9d on the substrate) in one exposure or xe2x80x9cshot.xe2x80x9d A promising current approach to performing exposure of entire patterns is to divide the reticle into multiple exposure units usually termed xe2x80x9csubfields,xe2x80x9d each defining a respective portion of the overall pattern, and exposing the subfields individually according to a preset order. This approach, termed xe2x80x9cdivided-reticlexe2x80x9d CPB microlithography, offers prospects of achieving the required resolution at a reasonable throughput.
In divided-reticle CPB microlithography, the reticle is mounted on a movable reticle stage, and the substrate (e.g., a semiconductor wafer) is mounted on a movable substrate stage. As noted above, the reticle is divided into multiple subfields that are exposed sequentially. Exposing subfields does involve a limited magnitude of lateral beam deflection. But, to expose all the subfields, coordinated movements of the reticle stage and substrate stage also are required. The coordinated movements also ensure that the respective images of the subfields are formed at respective locations on the substrate such that an image of the entire pattern is xe2x80x9cstitchedxe2x80x9d together properly in a contiguous manner from the individual subfield images.
The reticle stage and substrate stage are driven in each of the X- and Y-directions by a respective linear-induction motor (xe2x80x9clinear motorxe2x80x9d). (The direction of propagation of the charged particle beam is regarded as a Z-axis direction; hence, the reticle and substrate extend in respective X-Y planes.) The reticle stage and substrate stage, as well as their accessory parts such as the respective linear motors, are housed in respective chambers termed the xe2x80x9creticle chamberxe2x80x9d and xe2x80x9csubstrate chamber,xe2x80x9d respectively.
In a CPB microlithography apparatus, the charged particle beam must propagate in a xe2x80x9cvacuumxe2x80x9d environment (extremely low subatmospheric pressure). To such end, the various lenses, deflectors, diaphragms, and the like are contained in a xe2x80x9cCPB-columnxe2x80x9d which is essentially a vacuum chamber. The CPB-column typically includes a first portion housing an illumination-optical system and a second portion housing a projection-optical system. In addition, the reticle chamber and substrate chamber are maintained at respective vacuum levels.
A charged particle beam, even though it is contained within a CPB-column, can be affected by external magnetic fields, which can alter or disrupt the trajectory of the beam within the column and thus degrade pattern-transfer accuracy and/or resolution. In order to obtain the desired high level of pattern-transfer accuracy and resolution, various approaches have been considered for protecting the beam from external magnetic fields. According to one approach, the entire CPB microlithography apparatus is contained in a magnetically shielded room. In this regard, see Ogasawara et al., Teion Kogaku 8(4):135-147, 1973. However, because a CPB microlithography apparatus is a very large machine, the room containing it also must be large, and magnetically shielding a large room is difficult and very expensive. If the shielding is performed using a ferromagnetic material having a given thickness and a given number of layers, then the shielding factor decreases in inverse proportion to the room dimensions. Hence, with increased room size, the shielding material must be made very thick and/or configured with a large number of layers to achieve the same degree of shielding in the room.
A conventional linear motor used for driving a reticle stage or a substrate stage generates a large (several hundred mGauss) DC magnetic field as well as a fluctuating magnetic field of a few dozen mGauss. These magnetic fields can have a significant effect on the charged particle beam, which presents a substantial problem in machine design. Inside the CPB-column, the effect of these magnetic fields must be reduced to about 1 mGauss or less to achieve high transfer accuracy and resolution. But, heretofore, the best approach for achieving this goal has been unclear.
In view of the shortcomings of conventional approaches as summarized above, an object of the present invention is to provide charged-particle-beam (CPB) microlithography apparatus that can achieve high pattern-transfer accuracy and resolution without having to install the apparatus in a magnetically shielded room. Another object is to reduce the effects of magnetic fields, generated by the linear motors that drive the reticle stage and substrate wafer stage, on beam trajectory in a CPB microlithography apparatus.
To such end, and according to a first aspect of the invention, CPB microlithography apparatus are provided. An embodiment of such an apparatus includes an illumination-optical system and projection-optical system contained within a CPB-column. The embodiment also includes a reticle stage configured to hold a reticle during microlithographic exposure, and a substrate stage configured to hold a substrate during microlithographic exposure. Each of these stages is actuated by a respective linear motor. The reticle stage and its respective linear motor are enclosed in a reticle chamber that is situated between the illumination-optical system and the projection-optical system and is made of a ferromagnetic material. Similarly, the substrate stage and its respective linear motor are enclosed in a substrate chamber that is situated downstream of the projection-optical system and also is made of a ferromagnetic material. Situated in the reticle chamber is a first partition shield made of a ferromagnetic material and situated between the respective linear motor and the CPB-column. The first partition shield defines a gap allowing unobstructed movement of the reticle stage through the gap. Situated in the substrate chamber is a second partition shield made of a ferromagnetic material and situated between the respective linear motor and the CPB column. The second partition shield defines a gap allowing unobstructed movement of the substrate stage through the gap.
Desirably, each of the first and second partition shields comprises opposing lip portions having respective edges between which the respective gap is defined. Also, each partition shield desirably is attached to an inside wall of the respective chamber by a non-magnetic fastener.
Each of the reticle and substrate stages can be regarded as extending in a respective X-Y plane that is perpendicular to a Z-axis (the Z-axis normally is parallel to the optical axis of the illumination-optical and projection-optical systems). Hence, each partition shield can comprise a first portion extending in a respective X-Y plane on one side of the respective linear motor, and a second portion extending in a respective X-Y plane on an opposing side of the respective linear motor. With respect to each of the partition shields, the first and second portions desirably are attached separately to respective inside walls of the respective chamber by a non-magnetic fastener. Each of the first and second portions desirably defines a respective lip portion, wherein the lip portions are situated opposite each other in the X-direction, and the lip portions have respective edges between which the respective gap is defined.
Each of the partition shields desirably comprises multiple sheet layers of ferromagnetic material. Between each pair of sheet layers of ferromagnetic material is a respective sheet layer of non-magnetic material. This configuration is desired because it provides an enhanced magnetic-shielding effect compared to a shield consisting of a single sheet of ferromagnetic material.
With respect to each of the partition shields, the sheet layer of ferromagnetic material situated closest to the CPB-column desirably has a high relative permeability, and the sheet layer of ferromagnetic material situated closest to the respective linear motor desirably has a relatively high saturation magnetic flux density. This configuration is desirable because a linear motor produces a powerful magnetic field in the vicinity of the motor. Placing a ferromagnetic material having a high saturation magnetic flux density near the motor reduces the possibility of losing the shielding effect due to saturation of the shielding material. In contrast, the strength of the magnetic field is low nearer to the CPB-column, so the nearby shielding material need not have a high saturation magnetic flux density. Use of a high-relative permeability material instead in this location allows the material to be thinner, thereby reducing cost and bulk.
Another embodiment of a CPB microlithography apparatus comprises an illumination-optical system, projection-optical system, reticle stage, substrate stage, reticle chamber, and substrate chamber essentially as summarized above. The apparatus also includes at least one coil individually situated relative to at least one of the reticle chamber and substrate chamber. The coil is configured to circulate a magnetic flux through the ferromagnetic material of the respective chamber. Desirably, each of the reticle chamber and substrate chamber includes a respective coil, and the coils desirably are situated externally to the respective chamber. The coil desirably is configured to be energized by AC power biased by DC power, especially so as to achieve a maximal permeability of the ferromagnetic material of the respective chamber. With this embodiment, the relative permeability of the ferromagnetic material of the reticle chamber and substrate chamber can be adjusted to optimal respective values by adjusting the energization of the respective coil. Thus, a favorable magnetic shielding effect can be maintained in the chambers.
This second embodiment also can include at least one partition shield as summarized above.
According to another aspect of the invention, methods are provided (in the context of performing CPB microlithography of a pattern using a CPB microlithography apparatus) for reducing incursion of a magnetic field from the linear motor to the charged particle beam in the CPB-column. In the method, at least one partition shield, made of a ferromagnetic material, is provided. A respective partition shield is placed in each of the reticle chamber and substrate chamber that also contains a respective linear motor. The respective partition shield is situated between the respective linear motor and the CPB-column, and defines a gap allowing unobstructed movement of the respective stage through the gap. The partition shield can be provided with a first portion and a second portion. The first portion is situated so as to extend in a respective X-Y plane on one side of the respective linear motor, and the second portion is situated so as to extend in a respective X-Y plane on an opposing side of the respective linear motor. Each partition shield can be provided with multiple sheet layers of ferromagnetic material, wherein a separate sheet layer of non-magnetic material is interposed between each pair of sheet layers of ferromagnetic material.
At least one sheet layer of ferromagnetic material desirably has a high relative permeability, and at least one other sheet layer of ferromagnetic material desirably has a relatively high saturation magnetic flux density. In such an instance, the sheet layer having high relative permeability desirably is situated closest to the CPB-column, and the sheet layer having the relatively high saturation magnetic flux density desirably is situated closest to the respective linear motor.
This method can include the step of situating a coil relative to the respective chamber containing the linear motor, and configuring the coil to circulate, whenever the coil is electrically energized, a magnetic flux through the ferromagnetic material of the respective chamber. To such end, the coil desirably is energized by AC power biased by DC power.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.